A snowboard consists of a curved-up nose, a tail, a top, a flat bottom, and two longitudinally concave outside edges. Unlike a ski or a toboggan, a snowboard cannot be ridden flat on its bottom surface. When ridden flat, there is a strong tendency for the downhill edge to catch the snow, causing the rider to lose control. A snowboard is always tipped up on its uphill edge and balanced on that edge. The angle of tilt of the snowboard and a forward-rearward shift of the weight of the rider is used to control the snowboard. The turning characteristics of a snowboard are associated with the shape of the outside edge. The longitudinally concave shape of the edge forms a line of contact with the snow describing an arc. This arc guides the snowboard and its rider through a sweeping turn. The more the snowboard is tipped up on one of its concave outside edges, the tighter the turn.
One of the problems with a snowboard involves changing from riding on one edge to riding on the opposite edge. During this transition the snowboard is flat on its bottom surface, and both outside edges are contacting the snow. If this transition is not expertly timed, the downhill edge of the snowboard can catch the snow, causing loss of control and, possibly, a fall.
Another problem with a snowboard occurs when trying to run straight down a gentle hill on the flat bottom surface of the snowboard. In this situation both outside edges are contacting the snow, and the curvature of the left edge is suggesting a left turn, while the curvature of the right edge is suggesting a right turn. The slightest undulation in the terrain will tip the force balance in favor of one of the edges, unexpectedly turning the snowboard out from under the rider, causing the rider to fall.
In summary, a snowboard is unstable while on its flat bottom surface. A snowboard requires the strength and agility of the rider to balance it on one of its edges at all times. Expert judgment and timing in shifting edges while turning is critical. Snowboards cannot be ridden straight downhill. Because of the instability and consequent unpredictability of a snowboard, the learning process can be surprising, unpleasant, frustrating and sometimes even injurious.
To make snowboarding easier to learn and more enjoyable, several improvements over a conventional snowboard have been attempted. For example, in U.S. Pat. No. 4,974,868 to Morris (1990) the main improvement claimed is a convex bottom to "facilitate rapid changing between edges" and one or several perpendicular channels formed in the bottom of the snowboard to "allow snow to escape from under the snowboard". However, Morris does not address the issue of instability caused by the downhill edge catching the snow. In the Morris invention this problem is actually made worse, because the sides project beyond the bottom of the snowboard.
In U.S. Pat. No. 5,018,760 to Remondet (1991) the main feature claimed is an asymmetry of the snowboard for the purpose of equalizing left and right turning characteristics and a second set of inside edges provided to "reduce the push required to engage an edge." Unfortunately, the second set of outward-facing edges accentuates the instability of the Remondet snowboard.
In U.S. Pat. No. 4,083,577 to Ford (1976) the main feature is a convex cross-section of the running surface and a pair of elongated blades reaching down toward the lowest point of the convex running surface. In the Ford invention the convex cross-section of the running surface forms a rocker which in combination with the generous downward extension of the claw-like blades accentuate the probability of accidentally causing the downhill edge to catch the snow. Due to the inherent sharpness of the blade it penetrates and catches the snow, causing loss of control, in spite of the radius provided on the outside corner of the blade.
In all three cases, and in all other patents and designs which I have studied, I have not found any measurable attempt to understand or improve the stability of a snowboard while riding on its central bottom surface.